When a concentration measured by a sensor is lower than a predetermined threshold, it is judged that a liquid is any different type of aqueous solution, and also, it is judged whether or not the different type discrimination is adequate. When the discrimination is adequate, the number of different type discriminating times is counted up based on a predetermined counted value, and when the number thereof is equal to or larger than a determination threshold, the different type discrimination is determined. At this time, at least one of the predetermined counted value and the determination threshold is dynamically set based on a liquid level of a reducing agent tank, so that the counting and/or the different type discriminating determination is performed considering convection which is changed according to a level change to thereby discriminate with high precision whether or not the liquid is the different type of aqueous solution.